ModelExplanations.txt

There is a top level class called "CoopSystem" with one String id as primary key and contains the other classes (CoopUser,EventNotification, Docuemtn,Message, CoopJob).
CoopSystem will effectively be a singleton. In the serice class, we make sure in creating a new isntance or getting one that
we have only one instance with primary key "Main data"

CoopUser is an abstract class with the username as primary key. It has 2 child classes (Student and Employer).
CoopUser has a bidrectional relationship with Documents the user authored, 2 biderctional relationships with Message (sent and received)

Student has a uniderectional relationship to Document (to store the personal documents like CV and Transcript)
It also has 2 booleans indicating whether the student accepts his documents to be availabele on the system to to the employers who hired him.

Employer has a unidirectional relationship to Student indicating the students who interned with this employer.
Employer has also a unidrectional relationship to EventNotification (any EventNotification instance created will be made availabe to all employers)

CoopJob is an association class between Employer and Student. It allows each Employer/Student to have any number of internships with any Student/Employer.
CooJob has a unidrectional relationship to Document, storing the documents which are required for the coop.
CoopJob has an enum tye specifying whta is the state of the coop (started,finished, interrupted....)

The Document class has an enum type specifying what sort of document this is.

Message has a unidrectional relationship to Document (any message can have attachements, which will remain in the system even if the message is deleted later).

Message, CoopJob, EventNotification, Document also have a primary key.